Such shelves are supported by rollers, and when they are small and light they can easily be pushed and pulled directly by hand. But once size and weight increases, this becomes progressively more difficult, and it is necessary to have some means of assistance. It is not always viable to have some sort of motor or power assistance, and so the solution is to have a manually operable arrangement with a mechanical advantage. One example is a crank handle turning a spindle connected by sprockets and a chain to one of the rollers supporting the shelf. If the roller is at ground level, the spindle will be above it at about waist height for the user.
If the shelf is narrow, having this drive at just one side might be sufficient. But with a wider shelf, which will be correspondingly heavier, applying the drive to one side only will tend to skew it, and it might then bind or at least not run smoothly. Therefore, to cure this, as well as to give extra assistance in view of the increased weight, it is desirable to duplicate the drive system on the other side of the shelf.
However, there is then a synchronising problem. The spindles cannot be directly coupled by a co-axial shaft; that would traverse the load space above the shelf. So it is expedient to couple the drive rollers below the shelf.
Another problem with heavy shelves is that there are tremendous loads on the rollers, particularly when the shelf is fully out and most of the load is concentrated on just one pair of rollers almost at a point of balance. Conventionally, these rollers are carried by stub shafts inside uprights of a framework, and those shafts can be distorted under load. A cure for this is to extend the shafts through the rollers and provide another bearing on the inside of each roller. But this is not readily compatible with coupling the rollers.